Pads this string on the left if it is shorter than width.
Return a new string that prepends padding onto this string
one time for each position the length is less than width.
If width is already smaller than or equal to this.length,
no padding is added. A negative width is treated as zero.
If padding has length different from 1, the result will not
have length width. This may be useful for cases where the
padding is a longer string representing a single character, like
" " or "\u{10002}".
In that case, the user should make sure that this.length is
the correct measure of the strings length.
Source
String padLeft(int width, [String padding = ' ']);